1. Field of the Invention
The present invention relates to an apparatus for correcting the convergence of color television receivers, and more particularly to a digital convergence apparatus which is capable of accommodating various signal sources with a stable operation.
2. Description of Prior Art
Generally, in a projection color receiver for projecting an enlargement of a picture on its screen using three projection picture tubes emitting primary colors, the incident angle of the projection picture tubes to the screen varies with each tube, resulting in a color drift on the screen. Superposition of primary colors, or "convergence", has been adjusted in such a manner that it is allowed to synchronize with the horizontal and vertical scanning cycles to provide analogically a convergence correction waveform which in turn is allowed to be changed in size and shape. However, such method has a problem with respect to convergence accuracy. Accordingly, a digital convergence apparatus disclosed in Japanese Laid-open Patent Publication No. 60-130288/1985 has been proposed which can accommodate various signals and has a high convergence accuracy. Another apparatus disclosed in Japanese Patent Publication No. 1-26234/1989 has been proposed which determines the correction data of adjusting points outside the screen by an extrapolating operation.
Such prior art digital convergence apparatus will be explained hereinafter. FIG. 21 is a block diagram showing a prior art digital convergence apparatus which projects a pattern for convergence correction such as a crosshatch pattern (shown in FIG. 22) on a screen, writes digitally the data of convergence correction quantity for each adjusting point into a frame memory, and reads and D/A converts the data to perform convergence correction.
In FIG. 21, the element 9 is a crosshatch generator; element 13 is a video image circuit; element 50 is a read address controller; element 80 is a write address controller; element 1 is a control panel; element 110 is a reversible counter; element 90 is a multiplexer; element 6 is a one-frame memory; element 180 is a register; element 10 is a vertical interpolator; element 11 is a D/A conversion circuit; element 12 is an LPF; element 260 is a number of scanning lines detector; element 270 is a number between adjusting points setter; element 280 is a coefficient operator; element 14 is an output amplifier, and element 8 is a deflection circuit/high-voltage circuit. A horizontal/vertical cycle pulse synchronizing with a deflecting current cycle is added as a synchronizing signal to the read address controller 50, whereby the controller is driven. Utilizing the pulse from the read address controller 50, the crosshatch generator 9 is driven to project a crosshatch pattern on a projection screen. On the other hand, with the address key on the control panel 1, a cross point (for example, A in FIG. 22) of a position required for convergence correction is specified to set the position address to the write address controller 80. Then, with the data write key on the control panel 1 of a color you want to correct, for example, the color red, while seeing the screen, a correction quantity is written through the data reversible counter 110 into the one-frame memory 6. The writing into the one-frame memory 6 is generally changeover controlled by the multiplexer 90 so as to be performed during the blanking period of the video signal. Accordingly, no read is impaired.
Thus, similar operations at each adjusting point are performed. Then, the one-frame memory 6 is read by the read address controller 50 for each adjusting point on the screen, and the correction quantity in the vertical scanning direction between adjusting points is processed by the vertical interpolator 10 through the register 180 driven by the read address controller 50. In order to accommodate various signal sources, it is necessary to perform the processing between adjusting points corresponding to each number of scanning lines. Accordingly, an input synchronizing signal is supplied to the number of scanning lines detector 260 where the number of scanning lines of one field is detected and added to the number between adjusting points setter 270. In the number between adjusting points setter 270, the number of scanning lines N, N=M/(L+1), of the number between adjusting points is determined from the number of scanning lines M of one field and the number of adjusting points L in the vertical direction, and added to the coefficient operator 280. The number is also added to the write address controller 80 and the read address controller 50 change over to the operation for each N lines. The output from the vertical interpolator 10 operating as above is converted by the D/A conversion circuit 11 to a signal with an analog quantity. A signal between adjusting points in the horizontal direction is smoothed for the correction quantity of the adjusting point at each row by using the low pass filter (LPF) 12, amplified by the output amplifier 14, and then supplied to a convergence yoke 17. Also, a detection signal from the number of scanning lines detector 260 is added as a system changeover signal to the deflection circuit/high-voltage circuit 8 to change over the deflecting amplitude or frequency.
As described above, the prior art apparatus independently performs the correction for each adjusting point for various signal sources, so that it can perform convergence correction with a better accuracy.
However, the digital convergence apparatus having such an arrangement as described above can perform adjustment with a good accuracy for various signal sources during convergence adjustment, whereas it has a problem that it performs erroneous operation during non-signal period, or a receiver is damaged because of abnormal system changeover such as deflection. It also has a problem in that correction data input operation is required for various signals, resulting in a longer adjustment time. It further has a problem in that it cannot accommodate a signal source having a high horizontal scanning frequency because of a slow operation speed of the number between adjusting points setter.
It also has a problem in that the position of each adjusting point is hardly confirmed because the cursor display is dot blinking, and in that, because the cursor is displayed at the position of the extrapolating point outside the screen, the cursor display of the extrapolating point cannot be seen to make adjustments impossible when the deflecting amplitude of a projection television receiver is over scanned. It further has a problem in that the correction data of the extrapolating point is determined by the correction data from the adjusting points within the screen, so that the extrapolating points must be adjusted finally, whereby the current amplifying step to drive the convergence yoke requires an amplifier with a wide band. Such a current amplifying step necessitates a greater power consumption.